1) Field of the Invention
This invention generally relates to a metal-made honeycomb carrier body useful in an automotive exhaust gas cleaning device in order to carry an exhaust gas cleaning catalyst thereon.
More specifically, this invention provides a metal-made honeycomb carrier body, a principal component of an exhaust gas cleaning device of the above-mentioned type employed under severe conditions, which has been improved in durability against deformation and breakage which would occur by expansion and stress under heat.
2) Description of the Related Art
Conventionally-known catalyst carrier bodies for exhaust gas cleaning devices of the above-mentioned type include ceramic-made monolithic carrier bodies making use of a ceramic material such as cordierite and metal-made monolithic carrier bodies.
With a view toward improving certain drawbacks of ceramic-made monolithic carrier bodies, a great deal of research and development work is under way especially in recent years on metal-made monolithic carrier bodies from the viewpoints of mechanical strength, durability, flow resistance, purification efficiency (device size reduction) and the like.
A metal-made exhaust gas cleaning device of the above-mentioned type is generally constructed of a metal-made honeycomb carrier body and a cylindrical metallic casing. The metal-made honeycomb carrier body has been formed, for example, by superposing a planar metal band made of a heat-resistant steel sheet and a corrugated band made from a similar steel sheet one over the other in a contiguous relation and then rolling them into a spiral form or by stacking such planar bands and corrugated bands in layers, thereby defining a number of network-patterned gas flow passages along a central axis of the resulting carrier body. The network-patterned gas flow passages may hereinafter be called simply the "cells", while the metal-made honeycomb carrier body will hereinafter be called the "metal-made honeycomb carrier body" or simply the "honeycomb carrier body". The cylindrical metallic casing is open at opposite ends thereof so that the honeycomb carrier body can be inserted and fixed in the casing.
The honeycomb carrier body and the metallic casing are firmly fixed together by brazing or welding so that the resulting exhaust gas cleaning device can withstand thermal expansion and thermal stress, which occur because of the high temperature of exhaust gas itself and exothermic reactions of exhaust gas induced by a cleaning catalyst, and also vibrations during running of an associated automotive vehicle. Needless to say, the planar band and the corrugated band, which make up the honeycomb carrier body, can be fixed together at points of contact therebetween by various methods.
Exhaust gas cleaning devices, each of which is made of the above-described conventional honeycomb carrier body and a metallic casing, cannot however withstand long-term use for the reasons to be described next.
Based on thermal expansion and thermal stress which are produced in an atmosphere of the high temperature of exhaust gas itself and the heat generated through catalytic reactions of unburnt gas, significant deforming force occurs especially in a direction perpendicular to an axial direction (i.e., the direction in which exhaust gas flows in and passes) of the honeycomb carrier body, to say nothing of the axial direction. The direction perpendicular to the axial direction will hereinafter be called the "radial direction of the honeycomb carrier body". This deforming force propagates in both the axial and radial directions of the honeycomb carrier body and gives deleterious effects to the honeycomb carrier body. Described more specifically, such deleterious effects include so-called "filming out" of a central part of the honeycomb carrier body in the axial direction of the honeycomb carrier body, that is, the phenomenon that the central part of the honeycomb carrier body extends out to a downstream side relative to the remaining part of the honeycomb carrier body) as viewed in the direction of exhaust gas. In the radial direction, on the other hand, because of a temperature gradient between the central part of the honeycomb carrier body and its outer peripheral part, deforming force propagates close to the outer peripheral part of the honeycomb carrier body or close to faces of contact between an outer peripheral wall of the honeycomb carrier body and an inner peripheral wall of the metallic casing through the component members (the planar band and the corrugated band) of the honeycomb carrier body.
In the course of the propagation of the thermal deforming force, the component members of the honeycomb carrier body may be broken or buckled in the radial direction of the honeycomb carrier body and, because the thermal deforming force concentrates especially near the outer peripheral wall of the honeycomb carrier body or around faces of contact between the outer peripheral wall of the honeycomb carrier body and the inner peripheral wall of the metallic casing, the planar and/or corrugated bands forming the honeycomb carrier body at such places also undergo substantial cracking, breakage and buckling and separation or detachment takes place at points of contact between the respective bands or at faces of contact between the honeycomb carrier body and the metallic casing. Since the exhaust gas cleaning device is used under severe vibrations on an automotive vehicle as typified by a motor cycle, these drawbacks are aggravated further.
As measures for the improvement of the durability of such a honeycomb carrier body especially in the radial direction under thermal deforming force, many proposals have been made including the followings:
i) Japanese Utility Model Application Laid-Open (Kokai) Nos. SHO 61-66610 and 62-158117, Japanese Patent Application Laid-Open (Kokai) No. SHO 63-105221, etc. disclose a honeycomb carrier body having a conical concavity or a conical convexity at one end portion thereof, especially at the end portion located on an upstream side as viewed in the direction of exhaust gas.
They seem to be effective in making an exhaust gas stream uniform relative to the honeycomb carrier body or in preventing the so-called "filming out", but are not effective against buckling, cracking and/or breakage of the members forming the honeycomb carrier body (i.e., the planar band and the corrugated band) observed especially at the end portions of the honeycomb carrier body.
ii) Japanese Patent Application Laid-Open (Kokai) No. SHO 63-182038 discloses a technique for fabricating a honeycomb carrier body from a first corrugated band and a second corrugated band having mutually-different wavelengths (periods) .lambda. and waveheights (amplitudes) A. Namely, it is attempted to withstand radial pressure and expansion, which occur through thermal deformation cycles, by using a first corrugated band having a relatively large period and amplitude and a second corrugated band having a relatively small period and amplitude.
In the invention disclosed in the above patent publication, however, the first and second corrugated bands are not maintained in contact in a ridge-to-ridge (or groove-to-groove) relationship or in a ridge-to groove relationship (needless to say, the former relationship is preferred). Even if the first and second corrugated bands are fixed together by brazing at points of contact therebetween, they are caused to separate from each other under large thermal deforming force so that various drawbacks will be developed in association with such separation.
iii) Japanese Patent Application Laid-Open (Kokai) No. SHO 64-30651 discloses a technique for fabricating a honeycomb carrier body from a planar band and a corrugated band. The planar band has curved surface portions formed in advance so that the planar band can be brought into face-to-face contact with the curved surface portions (the ridges and grooves) of the corrugate band. The planar band may hence be regarded as a kind of corrugated band. A primary object is to bring both the bands into face-to-face (inner/outer) contact at their respective convex/concave surface portions, thereby making it possible to improve the strength of joining and also to save expensive .lambda.-alumina upon wash coating. Moreover, the planar band is in a wavy form as described above so that radial thermal deforming force can be reduced by the curved surface portions.
The invention of the above patent publication is however accompanied by the drawback that, because the planar band and the corrugated band are in face-to-face contact, the ability of the planar band to absorb and reduce thermal deforming force has been lowered.
In the conventional exhaust gas cleaning device of the above-described type, a common concept is employed especially to make its honeycomb carrier body sufficiently resistant to thermal deforming force, that is, to simply fix the planar band and the corrugated band firmly together, both the bands forming the honeycomb carrier body, or the outer peripheral wall of the honeycomb carrier body and the inner peripheral wall of the metallic casing firmly together. However this approach has to be modified.